Method of manufacturing a polycrystalline ferrite body

ABSTRACT

A method of manufacturing a polycrystalline ferrite body obtained by sintering a compressed block of the starting material. A readily detrition-resistant material having interdigitated crystals of an average grain size of more than 50 microns is obtained by adding prior to sintering a material from the group BaF2, SrF2; the oxides of B, Bi, Ca, Cu, Mg, Pb, Si, V; and Fe3(PO4)2 and choosing a very particular combination of added quantity and sintering temperature.

United States Patent [191 Van der Laan et al.

METHOD OF MANUFACTURING A POLYCRYSTALLINE FERRITE BODY Inventors: Karel Van der Laan; Hans Peter Peloschek, both of Emmasingel, Eindhoven, Netherlands Assignee: U.S. Philips Corporation, New York, NY.

Filed: Oct. 4, 1971 Appl. No.: 186,254

Foreign Application Priority Data Oct. 7, 1970 Netherlands 7014697 Aug. 18, 1971 Netherlands 7111359 U.S. Cl 117/234, 252/6258, 252/6259,

4 V r n 252/6264, 264/60 Hwww Field of Search ..252/62.5662.64;

[451 Sept. 24, 1974 [56] References Cited UNITED STATES PATENTS 2,992,990 7/1961 Parker 252/6259 X 3,002,930 10/1961 Robinson et al 252/6262 X 3,671,436 6/1972 Peloschek et al 252/6258 Primary Examiner-Jack Cooper Attorney, Agent, or Firm-Frank R. Trifari; Carl P. Steinhauser [5 7 ABSTRACT 4 Claims, 1 Drawing Figure PAIENItB Erzmn 3.8 87. 910

- C O+B O /0) INVENTORS KAREL VAN .DER LAAN HANS PETER PELOSCHEK BY GENT METHOD OF MANUFACTURING A POLYCRYSTALLINE FERRITE BODY The invention relates to a method of manufacturing a polycrystalline ferrite body, wherein a finely divided ferrite-forming starting mixture is formed, ground, pressed into a body having a desired shape and sintered.

A ferrite is to be understood to mean herein a crystalline reaction product of the oxides of iron and one or more other bivalent metals or bivalent metallic complexes.

Such a method is known from the US. Pat. No. 3,472,780.

As is known, bodies of sintered oxidic ferromagnetic material, for which material the name ferrite is generally used, are used inter alia as cores for electromagnetic transducers for converting electric signal variations into variations of the magnetic inductance, or conversely.

Since electromagnetic transducers, or magnetic heads, should cooperate in a number of applications in contact with a rapidly moving magnetic record carrier, for example a magnetic tape, the magnetic properties of the material and in particular the resistance to detrition, are of great importance. Two types of detrition are to be distinguished, the first type which is related to the hardness of the material used, manifests itself as a uniform overall reduction of the material of the contact surface of the transducer. The characteristics of the transducer in itself is not influenced by it. The second type which is inherent to the polycrystalline structure of the material used, manifests itself as a crumbling away of ferrite crystals out of the contact surface of the transducer under the influence of the scouring effect of the record carrier. When crystals crumble away at the edge of the effective gap, the rectilinearity thereof is lost and in circumstances the gap length, i.e., the dimension of the effective gap in the direction in which the record carrier moves relatively relative to the transducer, can be increased by it. It has been found actually that in the known regular crystal structures a crystal can crumble away comparatively easily along the boundary surface of the grain. Once in such a crystal structure one crystal has crumbled away, the surrounding crystals are supported and there is a fair chance that an increasing number of them starts crumbling away. The second type of detrition can therefore adversely influence the characteristic of the transducer.

This problem has been recognized in German Pat. No. 1,094,995 and the solution proposed in said patent specification as a core for a transducer which is to be readily resistant to crumbling away is the use of a ferrite body consisting of irregularly formed crystals which are as large as possible. In such a crystal structure the possibility of crumbling away of a crystal is much smaller since the irregularly formed and hence interdigitated crystals hold each other. It is proposed in particular in said patent specification to use a ferrite monocrystal as a core for a transducer. The manufacture of sufficiently large ferrite monocrystals, however, is expensive. Moreover, a drawback is that the resistance to detrition of the first type of monocrystals has been found to be not as large as that of polycrystalline material.

The invention provides a polycrystalline ferrite, having improved resistance to crumbling away. According to the invention a quantity of a material promoting grain growth or a combination of such materials chosen from the group consisting of BaF SrF the oxides of B, Bi, Ca, Cu, Mg, Pb, Si, V and Fe (PO is added to the ferrite-forming system during a manufacturing step preceding the sintering, such a combination of added quantity and sintering temperature being chosen, that interdigitated crystals of an average grain size of more than 50 microns are obtained.

In order to obtain a final product which as regards structure is as homogeneous as possible, according to a preferred embodiment of the method according to the invention the addition of the grain growthpromoting material or combination of such materials is made to the starting material to be ground. It is to be noted in that case the spreading in the average grain size may be very small, that is to say that so-called duplex structures do not occur.

In order to obtain a final product having magnetic and electrical properties which are favourable for its endeavoured application as a core for a magnetic head, the ferrite-forming starting mixture after grinding is preferably first pre-sintered and ground again before it is compressed to the desirable shape and sintered. In other words, a pre-sintered powder is used.

In order to obtain a final product having magnetic and electrical properties which are as favourable as possible, the method according to the invention may be varied so that the desirable crystal structure which is resistant to detrition occurs only at the surface or a part of the surface and that the internal structure is finegranular which is a condition for good electric and magnetic properties. A further preferred embodiment of the method according to the invention is characterized in that the ferrite-forming starting mixture is compressed and pre-sintered, if desired, that the grain growth-promoting material or combination of materials is locally provided on the outside of the body obtained after compressing and possible pre-sintering, and that said body is then sintered.

In addition, the method according to the invention may also be varied so that the final product as such has a fine-granular structure but shows paths or patterns which consist of interdigitated crystals having an average grain size of more than 50 microns. Another preferred embodiment of the method according to the invention is therefore characterized in that the grain growth-promoting material or combination of materials is added to the starting mixture, that same is ground, compressed and presintered, and that the body obtained after compression and presintering is locally heated, during sintering, at a sintering temperature such that the desirable crystal structure occurs only at that region. Local heating to the sintering temperature can be realized, for example, by means of a laser beam or by means of a Pt-wire through which a current is conveyed.

Certain grain growth-promoting materials have turned out to be very effective within the scope of the present invention, provided the correct combination of added quantity and sintering temperature is found.

According to a further embodiment of the method according to the invention during the manufacture of a Ni-Zn ferrite body from 0.001 to 1 weight percent is added of the mixture determined by its molecular composition: X8203 ySiO zCaO, with 1 s. X s 10; s y s 0 s z s 5, the sintering temperature lying between 1,180C and 1,275 C.

It has been found that in this manner by means of the above-mentioned addition, a ferrite body can be obtained having interdigitated crystals with an average grain size of more than 50 microns. It is to be noted that the spreading in the average grain size can be very small. This means that so called duplex structures do not occur. It is furthermore to be noted that the magnetic and electrical properties of the ferrite bodies obtained in this manner are acceptable in all respects.

Although the above-mentioned addition provides the desirable crystal structure, attention should also be paid to other properties. For example, the crystal boundaries should be clean and comprise no large pores. This can be achieved by causing the addition to be 0.007 to 0.25 percent by weight of the starting mixture.

According to a preferred embodiment of the method according to the invention, the addition is characterized by the following molecular composition: 213 0 SiO the sintering temperature lying between 1,180 C and 1,240 C. It has been found that, in particular after sintering of the compressed powder at a temperature between 1,220 C and 1',230 C, a product of maximum average grain size 500 microns) can be obtained.

According to another preferred embodiment of the method according to the invention, the addition is characterized by the following molecular composition: B 0 2Si0 CaO.

It has been found that, after sintering of the compressed powder, a product can be obtained in which the average grain size does not or only slightly depends upon the sintering temperature used. It has been found in particular that variation of the sintering temperature between 1,180 C and 1,275 C has hardly any influence on the average grain size, although said temperature lies below the values found in the preceding case.

According to a further embodiment of the method according to the invention, the addition is characterized by the following molecular composition: B 0 2SiO and the sintering temperatures lies between 1,240 C and 1,300 C. It has been found that, after sintering of the ground and compressed powder, a product is obtained having a combination of a high initial permeability 41., and a high density.

In manufacturing Ni-Zn ferrite bodies, the addition of Ca0 B 0 has furthermore turned out to be very effective. According to a preferred embodiment of the method according to the invention from 0.01 to 0.2 percent by weight of Ca0 B 0 is added during the manufacture of a Ni-Zn ferrite body and sintering takes place at a temperature between 1,175" C and 1,250 C, preferably between 1,200 C and 1,250 C.

Another preferred embodiment of the method according to the invention is characterized in that during the manufacture of a Ni-Zn ferrite body from 0.05 to 0.5 percent by weight of BaF and/or SrF is added and that sintering is carried out at the temperature between 1,200 C and 1,250 C.

Another preferred embodiment of the method according to the invention is characterized in that during the. manufacture of a Ni-Zn ferrite body from 0.05 to 0.5 percent by weight of BiF O is added and that sintering is carried out at a temperature between 1,175 C and 1,275 C.

The addition of Ca0 B 0 during the manufacture of Mn-Zn ferrite bodies has surprisingly been found to be even more effective than in the manufacture of Ni-Zn ferrite bodies, provided the correct sintering conditions where chosen.

Therefore, a preferred embodiment of the method according to the invention is characterized in that during the manufacture of a Mn-Zn ferrite body from 0.005 to 0.06 percent by weight of Ca0 B 0 is added and that sintering is carried out in an oxygencontaining atmosphere at a temperature between 1,350 C and 1,400 C.

A further preferred embodiment of the method according to the invention is characterized in that during the manufacture of a Mn-Zn ferrite body from 0.005 to 1 percent by weight of the mixture x B 0 y SiO zFe (PO,) whereinO x l;0 s y s 1;0 s 2 as 1 determined by its molecular composition is added and that sintering is carried out in an oxygencontaining atmosphere at a temperature between 1,350 C and 1,400 C.

Still a further preferred embodiment of the method according to the invention is characterized in that during manufacture of a Mn-Zn ferrite body from 0.005 to 0.5 percent by weight of BaF is added and that sintering is carried out in an oxygen-containing atmosphere at a temperature between 1,350 C and 1,400 C.

Another further preferred embodiment of the method according to the invention is characterized in that during the manufacture of a Mn-Zn ferrite body from 0.005 to 0.05 percent by weight of V 0 is added and that sintering is carried out in an oxygencontaining atmosphere at a temperature between 1,375 C and 1,400 C.

The invention also relates to a sintered oxidic ferromagnetic body manufactured by using one or more of the above-mentioned methods.

The invention will now be described with reference to the drawing and to the following examples.

EXAMPLE 1 To ferrite-forming systems of the composition 49.50 49.99 mol percent Fe O NiO and ZnO according to the proportion 18 32 was added a series of additions (0.01 percent by weight, 0.81 percent by weight and 1 percent by weight) of grain growth promoting materials.

Each of the resulting mixtures was pre-ground for 6 hours in a ball mill which also contained a grinding liquid and then prefired in oxygen at a temperature of 850 C for 3 hours. The mixtures were then postground for 16 hours. The powders obtained in this manner (grain size from 0.1 to a few microns) were precompressed to blocks at a pressure of 10 kg per cm and post-compressed in an isostatic pressure vessel at a pressure of 1,000 kg/cm The compressed product was sintered for 24 hours at temperatures of 1,180 C, l,200 C, 1,225 C, 1,250 C and 1,275 C, respectively, in a furnace containing an oxygen-containing atmosphere, for example air or a relatively more oxygencontaining atmosphere. The heating time was 16 hours and the cooling time was 24 hours.

The resulting products were polished and etched at one of their surfaces so as to evaluate their crystal structure. An average crystal size was determined with reference to microphotographs by dividing the distance covered during traversing by the number of crystal boundaries which is passed during covering said distance, in which of course the magnification standard 5 should be taken into account. I

TABLE I (molecular) composition of the by weight sintering temperature. PC) addition 1180 1200 1225 1250 1275 0.01 154 256 120 I64 256 1. 13,0, 0.1 111 272 246 113 62 1 55 53 62 65 0.01 271 230 679 104 37 2. 213 0 0.1 85 124 153 119 124 1 16 19 22 0.01 124 365 136 129 176 3. B 0 sio 0.1 76 82 100 95 95 1 9 14 14 24 34 0.01 19 18 23 165 250 4. B20, 2510 0.1 85 95 102 119 160 1 14 16 21 18 19 0.01 17 21 26 28 40 5. B 0,+2sio,+ 0.1 144 127 130 131 137 0 1 1o 10 11 13 21 0.01 13 20 22 29 6. sio CaO 0.1 70 76 71 68 54 1 2 34 112 153 161 0.01 23 83 95 75 7. 13,0, C210 01 110 110 154 84 1 23 33 36 17 0.01 12 15 19 43 8. Fe3(P0,)2 0.1 76 64 58 55 1 27 25 21 26 29 13 0 510 0.01 31 29 43 47 9. F6 00.)2 0.1 45 52 68 77 54 1 19 17 15 17 25 0.01 15 16 17 21 26 10. BaF- 0.1 3 121 111 73 86 1 7 13 15 16 14 0.01 13 16 18 24 28 11. si-F 0.1 3 68 161 58 45 1 11 13 16 21 32 0.01 25 20 20 28 35 12. CaF, 0.1 13 16 17 24 29 1 17 13 15 20 19 0.01 26 19 30 32 13. PbO 0.1 61 57 40 59 1 17 12 23 20 23 0.01 15 17 19 22 19 14. 131 0 0.1 93 92 81 100 90 1 36 43 30 26 50 When: It is to be noted that the sintered products obtamed by usmg the method accordmg to the 1nvent1on show A di i centimeters traversed on h h magnetlc and electnc values wh1ch are acceptable 1n all h, respects for the end 1n v1ew. The following example 3 m nifi ti d d 1 cm i demonstrates th1s: Add1t1on of 0.01 percent by we1ght N number of counted crystal boundanes, 50 2 3 2) Smtermg temperature glves a 8 average cross-section of a crystal in microns,

then 5 A/NXS (microns).

The average grain sizes found in this manner are represented in Table I as a function of the addition and of the sintering temperature for a sintering time of 24 hours.

On the basis of this table, an expert can adapt a distinct material to his specific wishes.

For comparison the average grain size of NiZn-ferrite manufactured in the usual manner is l0 to 20 microns,

product with a density5.305 g/ccm, and a p.',- L; and an addition of 0.01 percent by weight (B 0 2SiO sintering temperature l,250 C, gives a product with a density of 5.310 gloom and a p..- 1,700). (The X-ray density for these compositions is 5.336 g/ccm).

A series 6r more extensive investigations were carried out on ferrite-forming systems to which Ca0 B 0 had been added. The results of these tests are shown by the FIGURE which represents the average grain size (in microns) as a function of the added quantity in percent by weight and of the sintering temperature T. It is obvious that a maximum average crystal growth (250 ,u.) is obtained with a very particular combination of sintering temperature and added quantity.

It is to be noted that the result shown is characteristic of the behaviour of the grain growth-promoting materials mentioned further in this application.

It is furthermore to be noted that dependent upon the reactivity of the relevant ferrite-forming mixture (which is determined by the nature and the quality of the raw materials), the system of lines shown in the isostatic pressure vessel at a pressure of 1.000 kg/sqcm.

The compressed blocks were then sintered, in which the range of sintering temperatures, however, as

drawing of equal average grain size can slightly shift as chosen to be higher than in Example 1, namely from a whole relative to the axes. 1,200 to 1,400 C.

The avera e rain sizes measured in the final rod- EXAMPLE 11 g g P ucts are recorded 1n Table ll as a function of the nature A s1m1lar ser1es of experiments as in Example I was and quantity of the addition and of the sintering condicarried out in ferrite-forming systems of the following tions. composition: Condition A means that sintering was carried out at a temperature of l,300 C in air for 2 hours. 52'75 mol Fego Condition B means that sintering was carried out at 22 mol ZnO a temperature of 1,370 C in 100 percent oxygen for 5 25 mol Mno 5 hours so as to obtain the desirable crystal structure.

Condition C means that sintering was carried out at a temperature of L390 C in 100 percent oxygen for 5 The starting mixture was t eated I the Same manner hours so as to obtain the desirable crystal structure. as in Example I. This means that each mixture was pre- In the cases B and C, afterfining may then be carried ground in a ball mill for 6 hours and than pre-fired in 20 out at a lower temperature f example, {290 C) f air at a temperature of 850C for 4 hours. The mixture a few hours (for example 3 hours) in an atmosphere was then post-ground for 6 hours. The powders obcontaining 0.1 percent oxygen so as to adjust the ferrotained in this manner were pre-compressed to blocks at ferri equilibrium, after which conditioned cooling may a pressure of 10 kg/sq.cm and post-compressed in an be carried out, for example, in a nitrogen atmosphere.

TABLE II molecular composition of the 76 by weight sintering condition addition A B C 1 0.01 26 24 3 $10 (2110 0.1 81 16 1 24-300 24 0.01 828 16 4 8 0 1-810 0.1 130 169 14 1 44 21 1 0.01 648 710 5 B 0 +2SiO 0.1 12-130 240 481 1 61 339 0.01 764 931 6 213,0 +s1o 0.1 153 219 1 37 48 0.01 7 13 0 +2si0 0.1 90 282 373 CaO 1 22 0101 481 784 8 B O +SiO 0.1 255 23 FE1i O4 1, 1 68 117 0.01 1000 23 9 13. ,0 +2s1o 0.1 166 226 F600,), 1 17 89 0.01 994 1000 10 2B o +s10 0 368 355 Fett P0119 1 114 144 0.01 311 21 11 Fe (PO1)z 0.1 40-1000 1 22-450 321 0.01 96 1240 12 BttF 0.1 710 269 1 269 68 0.01 17 620 13 srF 0.1 14 211 1 304 47 0.01 24 23 14 CdF 0.1 17 66 1 35 48 0.01 18 23l80 15 131 0. 0.1 225 281 22-450 1 382 22-500 0.01 17 26 16 P130 0.1 347 21 1 261 24 0.01 23 19 17 C 01 27 174 1 22 271 0101 18 18 18 MgO 0.1 22 19 1 172 9 207 9 10 TABLE II CJntinued molecular composition of the by weight sintering condition addition A B C 0.0] 25 745 l V- .O 0.] 27 331 l 56 29 I8 24 20 0 2l 22 O 22 I8 EXAMPLE III A number of investigations were carried out on Ni-Zn ferrite samples on the surface of which a thin layer was provided (vapour-deposited or coated) of the crystal growth-promoting materials mentioned in the preceding examples, after which the samples were sintered at temperatures which varied from l,200 to 1,250 C and for periods of time which varied from 2 to 24 hours.

One half of the investigated samples were manufactured by grinding a (Ni-Zn) ferrite-forming mixture for 6 hours, presintering it at a temperature of 850 C in an oxygen-containing atmosphere for 3 hours, postgrinding for 16 hours, pre-compressing, isostatic postcompressing at a pressure of I00 kg/sq.cm, after which a layer of grain growth-promoting material was provided, and sintering for 24 hours in oxygen at a temperature of l,240 C. Samples manufactured in this manner turned out to yield no useful result.

The other half of the samples was manufactured by grinding a (Ni-Zn) ferrite-forming mixture for 6 hours, pre-sintering for 3 hours, post-grinding, precompressing and isostatically post-compressing at a pressure of 1,000 kg/sq.cm, after which a layer of grain growth-promoting material was provided on the surface. When the material to be provided is available in powder form, it can be provided directly on the compressed body. When the material is available in a liquid form, it is to be preferred to pre-sinter the compressed body slightly so that a denser structure is obtained.

The desired crystal structure occured without an exception after providing each of the above-mentioned grain growth-promoting materials followed by sintering. When using the materials CuO; CaO-l-SiO SrF BaF the desired crystal structure turned out to be restricted most to the surface and the least deformation of the surface also occurred.

EXAMPLE IV A number of detrition experiments was carried out both in magnetic heads of normal Ni-Zn ferrite having regularly shaped crystals and an average grain size of from 10 to microns, and in magnetic heads manufactured from Ni-Zn ferrite having interdigitated crystals having an average grain size of more than 50 microns manufactured while using the method according to the invention.

Commerically available magnetic tape (Kodak p 300 M; BASF-PES 18) were used for these experiments and Material Life Head regularly formed 1500 hours 5 10 to 20 microns irregularly formed 4500 hours 5 50 microns What is claimed is:

l. A method of manufacturing a detrition-resistant polycrystalline ferrite body comprising the steps of forming a finely-divided mixture of iron oxide, zinc oxide, and an oxide selected from the group consisting of nickel and manganese in proportions forming upon heating at an elevated temperature a nickel-or manganese-zinc ferrite, forming a body of said mixture, covering said body with a material promoting grain growth and selected from the group consisting of Ba F Sr F an oxide selected from the group consisting of B, Bi, Ca, Cu, Mg, Pb, Si, V, and Fe (P09 and thereafter sintering said body in an oxygen-containing atmosphere at a temperature of about l,l to 1,400C to form a coherent ferrite body having a surface structure of interdigitated crystals of an average grain size of more than 50 microns.

2. A method as claimed in claim 1 in which the body rial promoting grain growth is applied in liquid form.

4. A method as claimed in claim 1 in which the body is compacted by compression and the material promoting grain growth applied to the thus compacted body. l=

7% UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION Patent No. 1 m qm Inventor(s) KAREL VAN DER LAAN E1 AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' Column l,line 47 after "supported" insert -less-;

Column 2, line 12, after "regards" insert --crystal.,

Signed and sealed this 10th day of June 1975.

(3EAL) Attest C. MARSHALL DANN RUTH C. MASON Commissioner of Patents and Trademarks Arresting Officer 

2. A method as claimed in claim 1 in which the body is formed by compacting and pre-sintering said mixture at a temperature of about 850*C.
 3. A method as claimed in claim 2 in which the material promoting grain growth is applied in liquid form.
 4. A method as claimed in claim 1 in which the body is compacted by compression and the material promoting grain growth applied to the thus compacted body. 